Goshen, NY, USA
C-GGLM
Eurocopter AS-350BA
About 45 minutes into a cross-country flight, the helicopter experienced a hydraulic system failure. The pilot subsequently elected to perform a precautionary landing in a field. During the landing attempt, the pilot slowed the helicopter and it began to yaw left. The pilot attempted to correct the yaw, but was unable to overcome the "stiffness" of the right anti-torque pedal. The helicopter continued to yaw left, landed "hard," and rolled over. Examination of the helicopter's hydraulic system revealed that the hydraulic pump drive shaft splines and the attaching drive coupling splines were severely worn, to the point that they could no longer drive the hydraulic pump. Metallurgical hardness testing of both the drive shaft and the coupling revealed that the surfaces of the splines were below the manufacturer's specified hardness. Examination of the grease found trapped between the remaining portions of the hydraulic shaft pump splines revealed that it was hard packed and gray colored, suggesting that it had been in place for a significant period of time. The minimal amount of grease observed in the coupling sleeve did not satisfy the "generously filled" description referred to in the manufacturer's service bulletin, and required by a subsequent airworthiness directive. Examination of the helicopter's maintenance records revealed that since the issuance of the airworthiness directive, the helicopter had not been in compliance with either the calendar or flight time requirements. The helicopter's flight manual cautioned pilots operating the helicopter without hydraulic assistance, "Do not attempt to carry out hover flight or any low speed maneuver. The intensity and direction of the control feedback forces will change rapidly. This will result in excessive pilot workload, poor aircraft control, and possible loss of control."
HISTORY OF FLIGHT On May 29, 2006, about 1500 eastern daylight time, a Eurocopter AS-350BA, Canadian registry C-GGLM, was substantially damaged when it impacted the ground following an in-flight loss of control, near Goshen, New York. The certificated private pilot and passenger incurred minor injuries. Visual meteorological conditions prevailed for the personal flight, which departed Camden County Airport (19N), Berlin, New Jersey, destined for Glen Falls, New York. No flight plan was filed, and the flight was conducted under 14 CFR Part 91. In a written statement and during a telephone interview, the pilot recounted the events that transpired during the flight. About 45 minutes after departure, the warning horn sounded. The pilot noted that the main rotor RPM was normal, and that the "HYD" warning light was illuminated, which indicated a loss of hydraulic system pressure. The pilot actuated the hydraulic system cutoff switch on the collective pitch control handle, slowed the helicopter to 70 knots, and searched for a suitable landing area. The pilot selected a field and attempted a run-on landing. The pilot did not have any difficulty controlling the helicopter until it was about 30 feet above the ground. As he slowed the helicopter to an airspeed between a running landing and a hover, the nose began to yaw to the left. He attempted to correct by applying right anti-torque pedal, but was unable to overcome the pedal's "stiffness." The pilot again actuated the hydraulic system cutoff switch, but he still could not control the yaw, and subsequently lowered the collective to put the helicopter on the ground. The helicopter impacted the ground "hard" while moving backward and still yawing rapidly to the left, then rolled over. PERSONNEL INFORMATION The pilot held a private pilot certificate with a rating for rotorcraft-helicopter. The pilot's most recent third class medical certificate was issued on March 24, 2006, and on that date he reported 1,355 total hours of flight experience. AIRCRAFT INFORMATION The accident helicopter was manufactured in 1980. On March 7, 2005, the helicopter was converted from an AS-350D model to an AS-350BA model. The helicopter had accrued about 6,815 total flight hours. On January 27, 2004, the helicopter manufacturer released a service bulletin revision that in part modified the lubrication interval of the hydraulic pump drive shaft. The service bulletin recommended that operators reduce the lubrication interval from 550 fight hours, or 2 years, to 110 flight hours, or 6 months. It also described that during the lubrication, the mechanic should visually check the condition of the drive shaft splines. The drive shaft splines should then be lubricated and the coupling should be "generously" filled with grease before reinstallation. On April 27, 2004, a Canadian airworthiness directive (AD) was issued that in-part mandated a one-time replacement of the hydraulic system drive belt and a recurring lubrication of the hydraulic pump splines in accordance with the manufacturer's service bulletin issued on January 27. Examination of the helicopter's "engineering record" revealed that the Canadian AD was complied with during a 100-hour inspection, which was completed on April 13, 2004. The remark next to the entry read, "HYD Pump Belt Drive Replacement Due Sept 30/2004." The next recorded compliance with the AD occurred during a 100-hour inspection that was completed on August 25, 2005. Since the last logged compliance, the helicopter had accrued 204 flight hours. The remark next to the entry read, "HYD Pump Spline Grease C/O NDF." Examination of the traveling flight log that was recovered from the helicopter following the accident revealed a nearly identical record of maintenance entries and AD compliance; however, no record of compliance with the hydraulic pump lubrication AD was present. No subsequent records of compliance with the AD were located in either the engineering record or the flight record. METEOROLOGICAL INFORMATION The weather reported at Stuart International Airport (SWF), Newburgh, New York, about 13 nautical miles northeast of the accident site, at 1445, included variable winds at 4 knots, 15 statute miles visibility, and few clouds at 10,000 feet. WRECKAGE AND IMPACT INFORMATION A Federal Aviation Administration (FAA) inspector examined the helicopter at the scene. The hydraulic system reservoir contained fluid, and there was no evidence of a system leak. The hydraulic pump was secure, and the pump's drive belt was correctly installed and in good condition. The hydraulic pump and pulley assembly were removed, and examination revealed that the drive splines of the hydraulic pump were worn and "sawtoothed." There was evidence of "some lubrication" on the splines. The hydraulic pump and pulley assembly were forwarded to the Safety Board Materials Laboratory for further examination. TESTS AND RESEARCH The hydraulic pump was driven by a pulley assembly, which was in turn driven by a belt from the main gearbox input shaft. A splined shaft on the hydraulic pump was mated with a similar spline in a coupling sleeve that was located within the pulley assembly. Disassembly and examination of the hydraulic pump and pulley assembly components revealed no damage that would have prevented the pump from operating, with the exception of the hydraulic pump drive shaft splines and the mating pulley assembly coupling splines, which were both worn. Both the required drive shaft o-ring and coupling plug were installed. Examination of the drive shaft revealed that the normally flat crowns of the individual drive splines were narrow and rounded. The space between the crowns was packed with a hard gray substance. When the hard gray substance was removed, it retained its shape. Fourier Transform Infra-Red analysis of the substance revealed that it was consistent with the grease specified in the helicopter's maintenance manual for the lubrication of the splined connection between the hydraulic pump drive shaft and coupling. Metal and other contaminates were also contained within the grease. Removal of the grease revealed that material was missing from the driven flank of the spline, and a distinct ledge on the driven flank adjacent to the flank root. The amount of wear on the splines prevented any meaningful dimensional comparison with the requirements of the manufacturer's spline specification. Only the outside diameter of 0.3825-inch was recorded for comparison with the specified outside diameter of 0.3877 to 0.3897-inch. Specifications from the hydraulic pump manufacturer stated that the steels used to fabricate the drive shaft would be heat treated and case hardened to produce a surface layer of 61-65 Hardness, Rockwell, C (HRC), and a core hardness of not less than 39 HRC. Hardness testing performed at various depths from the surface of one spline root, between 0.004 and 0.040-inch, revealed hardness ranging from 46 to 58 HRC, with a core of 44 HRC. Examination of the coupling interior revealed that a minimal amount of grease was present. Approximately 1/16-inch of spline material remained intact at the aft end of the coupling, corresponding with the area not engaged by the hydraulic pump drive shaft. The engaged portion of the coupling displayed a negligible amount of remaining spline material. Specifications from the helicopter manufacturer stated that the splines were nitrited to achieve a minimum hardness of 64 HRC to a depth of 0.004 to 0.008-inch. Hardness testing performed at various depths from the surface of the coupling interior to between 0.002 and 0.020-inch, revealed hardness ranging from 48 to 38 HRC, with a core of 38 HRC. ADDITIONAL INFORMATION The AS-350BA Flight Manual described what actions a pilot should perform in the event of a hydraulic system failure. First, the pilot should calmly reduce collective pitch and adjust the airspeed to between 40 and 60 knots. Next, the pilot should cut off the hydraulic system pressure using the collective lever push button. The pilot should then make a flat approach over a clear landing area and land with slight forward speed. A revision to the flight manual also gave the following caution to pilots operating the helicopter without hydraulic assistance, "Do not attempt to carry out hover flight or any low speed maneuver. The intensity and direction of the control feedback forces will change rapidly. This will result in excessive pilot workload, poor aircraft control, and possible loss of control." Additionally, the manufacturer issued a "telex information," dated December 9, 2003, advising pilots about flight manual procedures in helicopters equipped with a single hydraulic system. The telex emphasized that certain maneuvers should not be performed without hydraulic power, whether in training or during an actual failure. This included the caution: "Do not attempt to carry out certain maneuvers without hydraulic assistance: e.g.: hover flight, pedal turns in hover flight, hover flight with a tail wind, translation in ground effect. In fact, in these cases, the intensity and direction of the control loads, in particular, the fore-and-aft control loads will change rapidly. This might surprise the pilot and delay his reaction and he might not be able to recover the proper flight attitude of the aircraft." The telex also emphasized the proper landing procedure while training for a hydraulic power system failure. It stated, "Make a flat approach, nose into the wind, and perform a no-hover slight running landing at a low speed (10 kt are sufficient)."
Improper maintenance of the hydraulic pump/coupling, which resulted in a failure of the hydraulic flight control system. Factors included the substandard materials of the hydraulic pump drive shaft and coupling, and the pilot's failure to maintain an adequate airspeed during the landing.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
